The subject of the present invention relates to a method of field-flow fractionation (FFF), and more particularly to the development of pressure-gradient fields for influencing material within such FFF systems.
FFF, disclosed previously in U.S. Pat. No. 3,449,938, is the descriptive term referring to a broad field of technology developed primarily for separation and characterization of macromolecules and particles. The technique has demonstrated a capability of dealing with an enormous mass range, including particle sizes varying from a molecular weight of 600 up to particles of 1 micrometer in diameter, a mass range of approximately 10.sup.9.
FFF takes advantage of the nature of viscous flow in narrow channels. Under laminar flow conditions, the velocity of flow approaches zero near the wall of the channel. Any solute or particle confined in this quiescent region near the wall will have its motion retarded relative to solutes distributed over the more active regions of the total flow cross section.
FFF employs an influencing field to partition the desired solutes into the quiescent wall regions of a narrow column. The field is applied along an axis perpendicular to the flow axis. As the strength of the field is increased, the solute is driven further and further toward the wall and its downstream motion is increasingly retarded. Different solutes will be retarded differentially because they will interact to a different degree with the field and/or they will exhibit a different level of diffusivity that will selectively oppose the induced drift toward the channel wall.
Subtechniques of FFF are characterized according to the kind of field employed. Many possibilities exist, including thermal gradients, electrical fields, and sedimentation fields developed by centrifugal forces. The rate of migration of solutes in such FFF systems depends on the magnitude and type of field, the channel dimensions, the solute-field interaction, and the solute-solvent diffusion coefficient.
Some disadvantages are inherent, however, in the conventional FFF system. In each case, the force field imposed on the channel flow is external and requires particular responsive characteristics inherent within the interacting particle. Obviously, electrically neutral particles will experience negligible influence under an electric field gradient. Furthermore, different particles will respond differently, or sometimes not at all, to heat, electrical or other forces.
Other limitations arise by reason of the single inlet/outlet flow channel. Such configuration restricts the apparatus to segregation applications of various solute types within the channel fluid, as described in the referenced Patent. Such arrangement precludes solvent exchange and controlled solute concentration and dilution.